Due to the depletion of petroleum-based fuels and chemicals and their detrimental impact on the environment, a transition to renewable fuels and chemicals is garnering considerable attention. ((a) U.S. Energy Information Administration/Annual Energy Review 2011, Washington, D.C. 20585. (b) Corma, A.; Iborra, S.; Velty, A. Chem. Rev. 2007, 107, 2411. (c) Chheda, J. N.; Huber, G. W.; Dumesic, J. A. Angew. Chem. 2007, 119, 7298; Angew. Chem. Int. Ed. 2007, 46, 7164.) Of these renewable sources, lignocellulosic biomass (cellulose, hemicellulose, and lignin) plays a crucial role. Lignin is a major component of non-edible biomass (15-30% by weight; 40% by energy). ((a) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Wechhuysen, B. M. Chem, Rev. 2010, 110, 3552. (b) Ralph, J., Encyclopedia of Life Sciences, Lignins, © 2007 John Wiley & Sons, Ltd.)
Lignin is also a cheap byproduct in the production of pulp and biofuel. It is one of the few naturally occurring sources of high-volume aromatics and therefore represents a potentially valuable feedstock for the production of organic chemicals. ((a) Collinson, S. R.; Thielemans, W. Coord. Chem. Rev. 2010, 254, 1854. (b) Grabber, J. H. Crop Sci. 2005, 45, 820.) While conversion of cellulose and hemicellulose into pulp and ultimately fuel has been extensively studied ((a) Philippidis, G. P.; Smith, T. K.; Wyman, C. E. Biotechnol. Bioeng. 1993, 41, 846. (b) Kumar, P.; Barrett, D. M.; Delwiche, M. J.; Stroeve, P. Ind. Eng. Chem. Res. 2009, 48, 3713), lignin is generally regarded as a byproduct. It is most typically burned to harness its thermal energy in any number of processes. (Gosselink, R. J. A.; Jong, E. de; Guran, B.; Abaecherli, A. Ind. Crops Prod. 2004, 20, 121.)
Beyond simple combustion, there are two basic approaches to lignin utilization in common use today. (Sarkanen, K. V.; Ludwig, C. H. Lignins, Occurrence, Formation, Structure and Reactions; WileyInterscience, New York, 1971.) The first approach is to exploit the properties of this natural polymer in carbon fibers, adhesives, concrete products, oil well drilling muds, as partial phenol replacements in phenol-formaldehyde resins, and in electronic circuit boards. The second approach is to convert the lignin polymer into simple low-molecular-weight organic chemicals. Processes for oxidation of lignin and associated model compounds have been the focus of extensive investigation. ((a) Cui, F.; Wijesekera, T.; Dolphin, D. J. Biotechnol. 1993, 30, 15. (b) Cui, F.; Dolphin, D. Bioorg. Med. Chem. 1995, 3, 471. (c) Herrmann, W. A.; Weskamp, T.; Zoller, J. P.; Fischer, R. W. J. Mol. Catal. A: Chem. 2000, 153, 49. (d) Crestini, C.; Caponi, M. C.; Argyropoulos, D. S.; Saladino, R. Biorg. Med. Chem. 2006, 14, 5292. (e) Partenheimer, W. Adv. Synth. Catal. 2009, 351, 456. (f) Zhang, J.; Deng, H.; Lin, L. Molecules 2009, 14, 2747.)
For instance, the world's supply of artificial vanillin is commercially produced by oxidation of lignosulfonate from spent sulfite liquor from essentially a single mill. (Hocking, M. B.; J. Chem. Educ. 1997, 74, 1059.) The process is via fairly simple oxidation, using molecular oxygen or stoichiometric CuO in aqueous basic solution. ((a) Larsson, S.; Miksche, G. E. Acta. Chem. Scand. 1971, 25, 647. (b) Pepper, J. M. Casselma, B. W.; Karapall, J. C. Can. J. Chem. 1967, 45, 3009.) The process, however, uses a lignosulfonate feedstock that arises from the expensive and more polluting sulfite pulping process, which is used in very few mills today. Also, oxidation of lignin with stoichiometric nitrobenzene in 1M NaOH at 170° C. to produce vanillin and syringaldehyde (up to ˜20%) is known. ((a) Yamamura, M.; Hattori, T.; Suzuki, D. S. Plant Biotech. 2010, 27, 305. (b) Nillar, J. C.; Caperos, A.; GarciaOchoa, F. J. Wood Chem. Technol. 1997, 17, 259.)) (See Scheme 1 for the structure of vanillin and syringaldehyde). This process, however, is not suitable for industrial use because of its high energy; thus it is not scalable. (The reaction can be explosive, even at laboratory scale.)

All of these methods suffer from environmental concerns, safety concerns and lack of structural specificity when starting from raw lignin. Therefore, identifying new chemical transformations of lignin that can proceed with high efficiency and selectivity is a long-felt and unmet need.
Lignin is a highly complex biopolymer having a variable structure. The variability of lignin's structure depends, at least in part, on its origin. The most common structural feature in all lignins is a β-O-4 linkage between aromatic rings (>85%; see Scheme 2). ((a) Ibrahim, W.; Lundquist, K. Acta. Chem. Scand. 1994, 48, 149. (b) Martínz, Á. T.; Rencoret, J.; Marques, G.; Gutiérrez, A.; Ibarra, D.; Jimenez-Barbero, J.; del Rio, J. C. Phytochemistry 2008, 69, 2831. (c) Vanholme, R.; Demedts, B.; Morreel, K.; Ralph, J.; Boerjan, W. Plant Physiology, 2010, 153, 895.) Another structural feature of lignin is aromatic rings containing secondary benzylic alcohol substituents and primary aliphatic alcohol substituents. Simple model compounds, such as 1, have been used to simulate the chemical reactivity expected from authentic samples of lignin.

Recent studies have begun to make progress in the development of catalytic aerobic oxidation of more realistic lignin model compounds, such as 1. For example, some research groups have identified several vanadium complexes that show promising aerobic reactivity, in several cases promoting multistep reactions that directly afford C—C/C—O cleavage products. ((a) Son, S.; Toste, D. Angew. Chem. Int. Ed. 2010, 49, 3791. (b) Hanson, S. K.; Baker, R. T.; Gordon, J. C.; Scott, B. L.; Thorn, D. L. Inorg. Chem. 2010, 49, 5611. (c) Hanson, S. K.; Wu, R.; Silks, L. A. Angew. Chem. Int. Ed. 2012, 51, 3410. (d) Zhang, G.; Scott, B. L.; Wu, R.; Silks, L. A.; Hanson, S. A. Inorg. Chem. 2012, 51, 7354.)
The irregular and complex structure of lignin, along with unsustainable approaches in the art, present profound technical, economic, and environmental challenges to using lignin and lignin-like materials as a bio-based chemical feedstock. Chemoselective depolymerization of lignin materials to value-added chemicals is a key step in this process. As discussed above, a number of homogenous and heterogeneous catalyst systems have been examined. However, all of these approaches suffer from insurmountable problems, such as non-selective transformations (which results in widely variable product mixtures), a huge array of low-yield products (thus making product separation difficult and/or impossible, as well as cost-prohibitive), prohibitive catalyst prices, and production of large quantities of unwanted, low-value by-products. Thus, there remains a long-felt and unmet need for a method to depolymerize lignin and lignin-like materials selectively, without the need for precious metal catalysts, and without requiring harmful, explosive, or otherwise difficult-to-handle reagents.